1. Field of the Invention
The present invention relates to a hemming apparatus and method for forming a hem on an edge of a sheet metal member and, more particularly, the invention relates to the forming of a hem on an edge of a structural sheet member such as an automobile body panel or similar multipart structure.
2. Description of the Prior Art
Sheet metal folding or hemming is a technique that has gained widespread use in many industries including the automotive industry. Hemming is utilized to form a piece of metal that serves as a reinforcing element for external automobile components such as doors and hoods. For example, the trunk lid for most automobiles is of two-piece construction in which the outer edge of the exterior element of the trunk lid is folded over against the outer edge of an inner reinforcing element by a hemming process.
The hemming procedures, as described in the prior art, utilize an outer element with the outer edge prefolded in the form of a flange to lie nearly perpendicular to the main portion of the outer element. Such preforming is most conveniently done in the stamping operation, that is, customarily utilized in the forming of such outer element. The hemming of such flange requires that it be folded over from such prefolded condition at approximately a right angle to be against the outer edge of the inner element after the inner element has been placed inside the upturned flange of the outer element. The folding over or hemming of the flange of the outer element in many hemming processes of the prior art is accomplished in multiple stages, usually in two stages. In a first stage, force was applied generally perpendicular to the original orientation of the flange to cause it to bend a considerable angle from its original orientation in which a second stage force was applied generally parallel to the original orientation of the flange to cause the partially bent flange to bend an additional amount to complete the folding of the flange from its prefolded condition to securely engage the outer edge of the inner element of the two piece structure that was being hemmed. Such two stage hemming process is done in separate sets of tools and the required tooling for such an operation is rather massive, costly, and space consuming. Additionally, a two stage hemming process requires a transfer operation to transfer the workpiece that is being hemmed from the first stage tooling to the second stage tooling. Such a transfer operation, which is generally synchronized, involves special transfer equipment and poses additional risks of equipment malfunction which can lead to interruptions on the production line. Multiple stage hemming operations of the aforesaid type also require for process consideration a certain minimum depth of flange in the outer edge flange of the outer element that exceeds the depth of the flange that would otherwise be required based on the product requirements of the component that is being hemmed. Then, too, the finished component is more costly and has a greater weight than would otherwise be necessary.
The present invention differs from the hemming tool that is shown and described in U.S. Pat. No. 4,706,489 entitled "Single Station Hemming Tool" issued Nov. 17, 1987 to Ernest A. Dacey, Jr. The hemming tool set forth in the above referenced patent utilizes a plurality of similar hemming tools that are spaced strategically around the perimeter of an automobile panel component. Each one of the hemming tools is similar in construction and is activated by a common actuator. The hemming tool has a flange contacting member and through a system of cams and levers the flange contacting tooling is initially driven generally perpendicular of the flange with respect to the original orientation of the flange of the outer element to accomplish a first stage hemming or prehemming of such flange and is subsequently driven generally parallel to the original orientation of the flange to complete the hemming or folding of the flange. As the hemming tool approached the flange of the component to be hemmed, the tool traveled along a first elliptical path, then a second elliptical path was utilized to complete the hemming operation. The mechanism for driving the hemming tool through a path with two elliptically arcuate portions included a cam that moved the center of movement of the hemming tooling from the center of the first ellipse to the center of the second ellipse at a predetermined point in the movement of the hemming tool that corresponds to the completion of the first elliptically arcuate movement. In order to accomplish the aforesaid compound elliptical movement, there was a heavy reliance upon a complicated cam and cam follower arrangement that was difficult to keep in adjustment and also was prone to unacceptable wear. The present invention does not use the cam system taught in the above referenced patent. Also, the present invention provides a greater freedom of movement of the tooling than what is described in U.S. Pat. No. 4,706,489.
The advantages of performing an entire hemming operation in a single stage is recognized in U.S. Pat. No. 3,276,409 entitled "Assembly Machine" issued Oct. 4, 1966, to Edouard R. St. Denis. This patent shows a hemming structure that employs several like units spaced around the periphery of a panel component that is to be hemmed. A pair of fluid driven cylinders is employed to drive the hemming tool of each unit toward the upstanding portion of an outer panel flange that is to be crimped or hemmed. The tooling first contacts the upstanding panel flange near the free end of the flange. The flange is then bent to nearly half of its final bend. The tooling then slides over and downward upon the already bent flange in order to hem the outer panel into tight engagement with the inner panel. At no time did the tooling leave contact with the outer panel flange, thus, undesirable stretching of the panel material was a distinct possibility. In the present invention, the tooling performs an initial bend then reorients itself remote from the flange before performing the final hemming step.
A somewhat similar hemming machine is shown and described in U.S. Pat. No. 3,191,414 entitled "Hemming Machine or Fixture" issued June 29, 1965, to James A. Kollar et al. The Kollar et al machine utilizes two fluid driven cylinders to move the hemming steel in a direction toward the anvil. The workpiece is placed on the anvil with the outer panel of the workpiece already bent to approximately a right angle. Through the action of one fluid driven cylinder, the hemming steel moves generally parallel to the surface of the anvil thus causing an additional bending of the upstanding flange to an acute angle configuration. The second fluid driven cylinder then causes the hemming steel to move toward the anvil surface thus completing the hemming operation. At no time does the hemming steel leave contact with the flange of the workpiece. The continuous contact of the tooling with the workpiece can in some instances produce undesirable stretch marks in the workpiece. The present invention is an improvement over the just cited art in that the hemming steel is completely reoriented after it makes initial contact with the workpiece.
In U.S. Pat. No. 4,484,467 entitled "Beaded Edge Forming Method and Apparatus" issued Nov. 27, 1984, to Mikio Kitano et al, there is set forth a method of hemming the flange of a workpiece. The method involves essentially two deforming steps. First, a deforming tool is moved vertically downward toward an upstanding workpiece flange, the downward tool movement deforms the flange until it is partially bent, and the deforming tool is then moved out of contact with the workpiece flange. Second, a hemming tool is moved into position above the partially bent flange of the workpiece, the hemming tool is lowered vertically against the workpiece flange thus crimping it into final position, and the hemming tool contacts only the outer portion of the workpiece flange thus preserving a previously formed bead at the base of the workpiece flange. The present invention employs a method that causes the hemming tool to disengage itself from contact with the workpiece, however, only one hemming tool is used whereas the method set forth in U.S. Pat. No. 4,484,467 requires two separate tools in order to perform the hemming method.